FIG. 6 shows a construction of a liquid crystal display apparatus in which a conventional surface light source device using a light conducting member is used as its back light system. In FIG. 6, the numerical reference 1 represents a plate-like light conducting member, which is made of a transparent material, 2 a linear light source disposed in the vicinity of a light emitting edge surface la of said light conducting member 1, 3 a light diffusing plate disposed on a front surface 1b of said light conducting member 1 and the numerical reference 4 represents a light reflecting member arranged on a rear surface 1c of said light conducting member, constituting of the surface light source device. Further, the numerical reference 5 represents a liquid crystal display apparatus.
In the surface light source device, a light emitted from the light source 2 is made incident upon the light emitting surface 1a of the light conducting member 1 and then the light is introduced inside thereof. During when the light is transmitted to an opposite edge surface 1d of the light conducting member, the light is emitted from the front surface (light emitting surface) 1b of the light conducting member and then passes through the light diffusing plate 3 to become a diffused light. The liquid crystal display apparatus 5 is illuminated by the diffused light coming from the light diffusing plate 3.
The illuminating light generated from the surface light source device is diffused in all directions by the light diffusing plate 3, but the most of them is directed to a direction shown by arrows in FIG. 6. Therefore, there exists some light which is not made incident upon the liquid crystal display apparatus, and thus enough brightness cannot be obtained on the apparatus. Further, out of the light being made incident upon the liquid crystal display apparatus, only a few light is directed to a direction substantially perpendicular to the liquid crystal display apparatus, because of the existence of the light diffusion plate 3. Therefore, there is a drawback that the image surface of the liquid crystal display apparatus is dark for an operator who observes the image surface in front of the liquid crystal display apparatus.
In order to dissolve the drawback, it is considered that a prism sheet 11, on the surface of which a multiplicity of prism-like portions are formed, is disposed between the light diffusing plate 3 and the liquid crystal display apparatus 5, and thereby the diffusing light emitted from the light diffusing plate 3 is directed to a direction which is perpendicular to the surface of the liquid crystal display apparatus. However, an enough effect cannot be still obtained by using such prism sheet having the prism-like portions, which are formed thereon in a symmetrical manner.
Further, it is also considered that a light diffusing plate 3' having a strong light diffusing power is used as shown in FIG. 7. In such a device, the light coming through the light diffusing plate 3' is diffused into directions shown by the arrows in an enough manner, so that it can be prevented that the large amount of the diffusing light is directed into a perspective direction as shown in FIG. 6.
However, in this device, since the light diffusing function is strong, the light directed in a perpendicular direction with respect to the light diffusing plate is relatively decreased, too. Therefore, the problem cannot be improved that the brightness is not enough for the operator who observes the image surface in front of the liquid crystal display apparatus. In this case, even if the prism sheet 10 is arranged in the device, it is impossible to increase the brightness thereof.
The present invention has its object to provide a surface light source device using a light conducting member, particularly a surface light source device having a highly qualified brightness, by which an observation can be carried out on a bright image surface when the surface light source device according to the present invention is used as a back light system of a liquid crystal display apparatus.